Say something funny or keep quiet! (3)
I believe the answer is:
gag
'quiet' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for gag that I've seen before include "Shut up" , "Choke; joke" , "Prevent from speaking freely" , "Joke; silencer" , "Stifle; joke" .)